The invention relates generally to a method and apparatus for driving thin materials, and in particular to a method and apparatus for precisely controlling the movement of film materials between a pair of rollers in an electronic imaging system.
In a high resolution system where film material is advanced for exposure to a laser beam one line at a time, it is important to precisely control the advancement of the film. When advancing the film between two rollers, if the film is misaligned by even 1/8000 of an inch, the recorded image may display unwanted artifacts and be flawed. Therefore, it is important to ensure that the film is driven uniformly along its entire length.
Various combinations of driven and pinch rollers have been used for driving film materials. The construction of such rollers can include covering one or both rollers with a rubber layer to grab the film. When the rubber covering is very soft, it advantageously grasps the film; however, the rubber can pull or stretch, causing the film to move unevenly, especially in the driven direction. When a hard rubber is used, the rubber may not grip the film firmly enough and slippage may occur. Other roller combinations pair a soft rubber-coated driven roller with a hard rubber-coated non-driven roller; but in such a compromise the film tends to curl around the soft rubber roller rather than properly exiting from between the roller pair. Pairing a metallic surfaced non-driven roller with a hard rubber-coated driven roller proved more successful; but friction between the rollers and the film was often insufficient, and some slippage can occur. Furthermore, in any of the above combinations, minute differences in the outside radius from one end of a roller to the other can cause the film to skew when driven.
Other prior solutions to the problem attempt to align the rollers by applying equal or separate pressure to the non-driven roller in order to control the radius of the driven roller. This solution has met with some success but results in a mechanically elaborate and expensive system.
A primary object of the invention therefore is to construct a roller system wherein one roller is urged against another roller in a manner which precisely controls the location of the film, produces a precise drive control, eliminates skew, and is mechanically simple to build.